1. Field of the Invention
The present invention relates to a defect inspection method and apparatus, which are suitably applied to inspections of defects such as foreign matters, which become attached to the surface of a mask or reticle (to be collectively referred to as a "reticle" hereinafter) used in the exposure process in the manufacture of, e.g., semiconductor elements or the surface of a pellicle (anti-dust film), which is formed to be separated by a predetermined interval from the surface of the reticle.
2. Related Background Art
When defects such as foreign matters are present on the surface of a reticle for exposure used in the manufacture of, e.g., semiconductor elements, using a photolithography technique, the manufacturing yield of the semiconductor elements is lowered. In order to avoid this, the states of defects on the reticle surface are conventionally inspected, before exposure, using a defect inspection apparatus. In order to prevent foreign matters from becoming directly attached to the pattern formation surface of the reticle and its rear surface, an anti-dust film called a pellicle is often extended on the surface of the reticle. Since such a pellicle is in a defocus state from the pattern formation surface of the reticle conjugate with the exposure surface of a photosensitive substrate (e.g., a wafer), if foreign matters have the same size, their influence if attached to the pellicle is smaller than if directly attached to the reticle.
However, when the size of foreign matters exceeds a predetermined limit, even foreign matters attached to the pellicle influence the exposure result. As for the front surfaces and rear surfaces (the surfaces on the side of the reticle) of the pellicle, the states of defects such as foreign matters are inspected by the defect inspection apparatus. In the following description, a reticle is used as an object to be subjected to defect inspection, and the reticle is assumed to include one on which a pellicle is formed.
FIG. 7 shows an example of a conventional defect inspection apparatus. Referring to FIG. 7, a reticle 1 is placed on a stage 2, and the stage 2 is movable in the Y direction by a driving unit 3. The moving amount, in the Y direction, of the stage 2 is always measured by a distance measurement device 4 such as a linear encoder, and a position signal S1 indicating the distance measurement value of the distance measurement device 4 is supplied to a signal processing circuit 5. In addition, a light beam L1 emitted from a light source (not shown; e.g., 8 laser light source) is reflected and deflected by a galvano scanner 6 (or, e.g., a polygonal scanner), which is vibrated by a driving unit 7. The deflected beam L1 is converted into a light beam L2, which converges on the reticle 1, via a scanning lens 8, and scans, in the X direction, a scanning line 10 substantially parallel to the X direction on the reticle 1. When the light beam L2 is scanned in the X direction, and the reticle 1 is moved in the Y direction by the driving unit 3 at a speed lower than the scanning speed, the entire surface of the reticle 1 can be scanned with the light beam.
If a defect such as a foreign matter 11 is present on the surface of the reticle 1, scattered light L3 of the light beam L2 from the foreign matter 11 is generated. The scattered light L3 is focused on the light-receiving surface of a photoelectric detector 13 such as a photomultiplier via a light-receiving lens 12, and a detection signal S3 obtained by photoelectrically converting the focused light in the photoelectric detector 13 is supplied to the signal processing circuit 5. The signal processing circuit 5 also receives a deflection angle signal S2 supplied to the driving unit 7 for the galvano scanner 6, and can determine the presence of the foreign matter 11 on the basis of the detection signal S3. Parallel to this processing, the signal processing circuit 5 can recognize the position of the foreign matter 11 on the basis of the position signal S1 from the distance measurement device 4 and the deflection angle signal S2 for the driving unit 7 from the galvano scanner 6 obtained when a signal indicating the foreign matter 11 appears in the detection signal S3. More specifically, the X-coordinate of the foreign matter 11 can be detected from the deflection angle signal S2, and the Y-coordinate of the foreign matter 11 can be detected from the position signal S1.
Since the amount of the scattered light L3 becomes larger as the size of the foreign matter becomes larger, the magnitude of the detection signal S3 from the photoelectric detector 13 indicates the size of the foreign matter. For this reason, the signal processing circuit 5 can display the attached position (X, Y) and size of the foreign matter on a CRT display 14 in the form of, e.g., a table. Alternatively, the signal processing circuit 5 can display the coordinates (X, Y) and size of the foreign matter, which are detected simultaneously with scanning of the light beam on the reticle 1, on the display screen of the CRT display 14 in the form of a two-dimensional map. Furthermore, after the position (X, Y) and size (the value of the detection signal S3) of the detected foreign matter are stored in a storage unit such as a memory in the signal processing circuit 5, the stored position and size can be read out from the storage unit after the end of inspection, and can be displayed on the CRT display 14 in the form of a two-dimensional map or a table, or can be printed out by a printer (not shown).
FIG. 8 shows a display example of a map on the CRT display 14. In this display example, the surface of the reticle 1 shown in FIG. 7 is divided into a large number of small rectangular regions (to be referred to as "cells" hereinafter), and foreign matter information on the entire surface of the reticle 1 is displayed in a rectangular window 17 on the display screen in FIG. 7 in units of cells. More specifically, as shown in FIG. 7, the surface of the reticle 1 is partitioned at predetermined pitches in both the X and Y directions to be divided into a large number of small (1- or 5-mm square) cells C(1, 1), C(1, 2), C(1, 3), . . . , C(2, 1), . . . The window 17 on the display screen in FIG. 8 is divided into display cells P(1, 1), P(1, 2), P(1, 3), . . . , P(2, 1), . . . in one-to-one correspondence with the cells on the reticle 1 in FIG. 7, and symbols A, B, and C indicating defects such as foreign matters are displayed in units of display cells.
In this case, the X and Y directions in FIG. 7 respectively correspond to X1 and Y1 directions in FIG. 8, and defects are displayed on display cells P(i, j) corresponding to cells C(i, j) to which coordinates (X, Y), where defects are detected, respectively belong, while being classified into ranks such as symbols A, B, and C in correspondence with the signal strengths of the detection signal S3. The symbols A, B, and C are ranks representing the sizes of defects. For example, when the detection signal S3 is small, a rank "A" representing a small defect is displayed; when the detection signal S3 is large, a rank "C" representing a large defect is displayed.
It should be noted that the following considerations apply to the defect inspection process.
(i) When detection signals S3 indicating foreign matters are obtained at two different coordinate positions within a single cell, the signal intensity of the larger detection signal. S3 and the coordinate position at that time are adopted as defect data in that cell. Normally, a larger defect poses a problem. For example, when a detection signal with a value "50" and a detection signal with a value "100" are obtained at different positions in a 1-mm square cell, the signal with the value "100" and the coordinate position where this detection signal is obtained are used as defect data of the cell.
(ii) Since the detection signal S3 includes electrical signal components and noise components due to very weak light other than scattered light from the foreign matter, a detection signal S3 having a value equal to or larger than a predetermined threshold value is used as defect data.
Another example of the conventional defect inspection apparatus will be described below with reference to FIG. 9. Referring to FIG. 9, a reticle 1 is placed on a stage 2, and the stage 2 is moved in the Y direction by a driving unit 3. The moving amount, in the Y direction, of the stage 2 is measured by a distance measurement device 4. A light beam L4 emitted from a light source 16 such as a laser light source is converted into a substantially collimated slit beam L5 by a lens system 19 comprising a cylindrical lens 18 and a collimator lens 18, and the slit beam L5 is obliquely irradiated onto the reticle 1. For this reason, on the reticle 1, a slit-shaped irradiation region 20 parallel to the X direction is irradiated by the slit beam L5.
Of course, when a defect such as a foreign matter 21 is present in the slit-shaped irradiation region 20, scattered light L6 is generated from the defect, and forms a defect image on a one-dimensional image pickup element 23 such as a one-dimensional CCD. In this case, the slit-shaped irradiation region 20 and the image pickup surface of the one-dimensional image pickup element 23 have a substantially optically conjugate positional relationship via a light-receiving lens 22. Therefore, in the apparatus shown in FIG. 9, the Y-coordinate of the attached position of the foreign matter 21 is measured by the distance measurement device 4, and the X-coordinate of the attached position is identified based on the pixel number of the one-dimensional image pickup element 23 on which the optical image of the foreign matter is formed. Furthermore, the sizes of foreign matters can be classified into ranks in correspondence with the strength of a detection signal S4 as a photoelectric conversion signal (image pickup signal) obtained from each pixel of the one-dimensional image pickup element 23. For this reason, the same map as in the apparatus shown in FIG. 7 can be displayed on a display device such as a CRT display.
More specifically, in the apparatus shown in FIG. 9 as well, the surface of the reticle 1 is divided into a large number of cells C(1, 1), C(1, 2), . . . , and a window 17 in FIG. 10 corresponding to the entire surface of the reticle 1 on the screen of the display device is divided into display cells P(1, 1), P(1, 2), . . . in correspondence with cells on the reticle 1. The states of defects are displayed on the display cells while being classified into ranks A, B, and C.
In in the above-mentioned prior art, when a large foreign matter is present on the reticle 1, defects are successively displayed as if two foreign matters were present adjacent to each other like on display cells P(2, 5) and P(2, 6) and display cells P(5, 6) and P(5, 7) in the window 15 as the display map, as shown in FIG. 10. Such a display error tends to occur when the size of the cell C(i, j) on the reticle 1 is small. More specifically, display errors occur more easily in the case of 1-mm square cells than in 5-mm square cells.
The defect inspection apparatus is often provided with an observation unit 15 shown in FIG. 7 to allow observation of defects. Referring to FIG. 7, the observation unit 15 is attached to a slider 16 to be slidable in the X direction. As the observation method of the observation unit 15 itself, a visual observation mode for observing defect portions on the surface of the reticle 1 using an optical microscope is available, or as another observation method, an image observation mode for picking up an optical image obtained by the observation unit 15 using a two-dimensional image pickup element (e.g., a CCD), and displaying the picked-up image on a TV monitor is available. In general, even when a foreign matter having a predetermined size becomes attached onto the surface of the reticle 1, if the attached position corresponds to a light-shielding portion coated with, e.g., a chromium film, the influence on the exposure result is small. However, if the attached position corresponds to a light-transmitting portion, the influence on the exposure result is large. More specifically, even when foreign matters having the same size become attached to the surface of the reticle 1, the use of the reticle 1 is enabled or impaired depending on the attached positions of the foreign matters. For this reason, by observing defects detected upon light beam scanning using the observation unit 15, whether or not the reticle 1 can be used is finally determined.
When the observation unit is arranged in this manner, a 1- or 5-mm square cell size, and in some cases, a 0.1-mm square cell size are used as the size of the cells C(i, j) on the reticle 1 in correspondence with the size of the observation field of the observation unit (when various observation magnifications such as low, medium, and high are available, the size of the observation field at a low magnification used for detecting a defect within the field). Therefore, in the case of a large foreign matter having a considerable area, a defect may be detected on a plurality of cells on the reticle 1. When the foreign matter is attached near the boundary between neighboring cells, it may be detected at the two neighboring cells, and the same defect ranks may be displayed for the neighboring cells.
In the conventional defect inspection apparatus comprising the observation unit, defect detection by means of light beam scanning and observation of defect portions using the observation unit 15 are performed. However, the defect detection by means of light beam scanning and observation of defect portions cannot be simultaneously performed. The two main reasons therefor are as follows:
(i) Although the defect detection is performed by an oblique incident method for obliquely irradiating the light beam L2, as shown in FIG. 7, the observation of defect portions using the observation unit 15 is attained by vertical irradiation or transmission illumination. For this reason, a common light source cannot be used for these two modes. (ii) In the defect detection, light other than very weak scattered light from a defect such as a foreign matter disturbs inspection. For this reason, when the observation unit 15 is arranged near the scanning line 10, the oblique incident beam (light beam L2) or scattered light of the light beam L2 from the surface of the reticle 1 is irradiated onto the observation unit 15 and excessive light is generated. Therefore, the observation unit and the defect detection unit must be arranged to be separated away from each other.
As described above, in the conventional defect detection apparatus, the detect detection and the observation of defect portions cannot be simultaneously performed, and the defect detection by means of light beam scanning is performed at high speed upon scanning of the galvano mirror 6 and movement of the stage 2. Therefore, most of the inspection time is a defect observation time using the observation unit 15, and in order to shorten the time required for the inspection process, the defect observation using the observation unit 15 must be performed quickly. However, the conventional inspection method merely allows observation of detected defects in the order of, e.g., detection on the reticle 1, and does not sufficiently consider means for shortening the time required for the inspection process.